The present invention relates generally to methods and apparatus for seismic exploration systems and more particularly to borehole deployed seismic systems. Still more particularly, the present invention relates to methods and apparatus for generating seismic, acoustic, or other signals within a borehole.
High-resolution seismic formation evaluation methods have been enabled by technology such as three-dimensional vertical seismic profiling (3-D VSP) and crosswell seismic tomography. These technologies can provide high-grade imaging data gathered from certain formations and fluids within a reservoir. This imaging data can be used by an operator to target areas within the formation most likely to produce hydrocarbons, thus resulting in improved production at reduced costs.
Borehole seismic surveys have been available since the late 1970's in the form of vertical seismic profiles (VSP) that are acquired using surface energy sources and receivers deployed in a single wellbore. Receiver tools have also been deployed in the wellbore that allow acquiring large 3-D VSP data sets from a grid of source points on the surface received by a plurality of receiver levels in the wellbore. In certain situations, such as those with environmental restrictions on surface source placement, a reverse 3-D VSP system may be used where the seismic receivers are deployed in an array at the surface and the seismic source is deployed within the wellbore. Large subsurface volumes can be interrogated using 3-D VSP to provide high-resolution imaging of a volume around a wellbore.
Crosswell seismic technology performs the seismic survey from within the reservoir by deploying a seismic source in one wellbore and deploying seismic sensors into one or more adjacent wellbores in the same reservoir. High-bandwidth data is collected between the wells, directly across the reservoir, or other zone of interest. Crosswell seismic systems can provide greater vertical resolution than is possible with surface seismic information. The measurements taken with crosswell technology can also be directly referenced in depth, allowing correlation with well logging data.
Borehole seismic sources are used routinely in crosswell surveys but have not realized their potential in reverse VSP because they are generally low-powered, high-frequency, or unreliable. Commercially available seismic sources can be categorized as impulsive sources or swept frequency sources. Impulsive sources, such as bolt air guns, sparkers, and explosives, generate high power but can cause damage to casing and cement. Impulsive sources also limited by depth restrictions.
Current swept frequency source technology falls into two categories, fluid-coupled and clamped. The fluid-coupled sources are operationally simple and cause no borehole damage, but suffer from low output, particularly at low frequencies. In addition, fluid-coupled sources must be operated in fluid filled wells, thus limiting their use in gas reservoirs. Clamped sources provide improved performance at low frequencies but can cause casing damage and are generally more complicated and expensive to deploy and operate.
Accordingly, it would be desirable to have a borehole seismic source providing high power and good low frequency response while being less complex, more reliable, and less likely to damage the casing. Thus, there remains a need to develop methods and apparatus for generating a seismic signal in a borehole, which overcome some of the foregoing difficulties while providing more advantageous overall results.